The hairpin ribozyme is an endonucleolytic RNA motif 50 nucleotides in length. The naturally occurring cis-acting ribozyme can be truncated and converted to act in trans by the deletion of sequences that are not required for substrate recognition and catalysis. To explore structure-function relationships and to investigate their use for selective inhibition of mammalian gene expression, several groups have used trans-acting hairpin ribozymes. The Burke laboratory has been conducting elegant fluorescence resonance energy transfer studies of the folding of the hairpin ribozyme that have provided information about the global motions of the domains of the RNA over the course of the folding pathway. These data complement the synchrotron x-ray footprinting studies that will monitor the formation of the individual tertiary contacts during folding. This availability of the fluorescence data together with the small size of the ribozyme, make this system ideal for further exploring the synchrotron footprinting technique. Preliminary studies are underway to determine the conditions under which synchrotron x-ray Hfootprinting studies of the folding of a hairpin ribozyme can be Hconducted.